Social systems theory has been dominated in recent years by the work of Niklas Luhmann, but there is another strand of systems thinking, which is receiving increasing attention in sociology: emergentism. For emergentism, the core problems of systems thinking are concerned with causation and reductionism; for Luhmann, they are questions of meaning and self-reference. Arguing from an emergentist perspective, the article finds that emergentism addresses its own core problem successfully, while Luhmann's approach is incapable of resolving questions of causation and reductionism. On the other hand, neither paradigm yet has a convincing response to the challenges of meaning and self-reference. Key Words: social systems  emergence  paradigms  autopoiesis  critical realism.